Discussion on Modulation Methods for Flyback-type Single-Phase Inverters with Enhanced Power Decoupling for Photovoltaic AC Module Systems

Abstract

Numerous number of photovoltaic (PV) power generation systems have been expected to be installed on the private residences. In order to make PV power generation systems ease to install, a concept of photovoltaic AC module system has been proposed. An AC module consists of a PV panel and a small power grid interactive inverter on it. The AC module inverters are required to be installed on a roof at high temperature condition. Hence, electrolytic capacitors, which provide large capacitance, can not be used on the input DC-bus line because the lifetimes of electrolytic capacitors are short at high temperature condition. In the case when a film capacitor, which has a long lifetime at high temperature condition, is used as the DC-bus input capacitor, its small electrostatic capacitance causes low frequency voltage pulsation on the DC input bus line and results in the difficulty of Maximum Power Point Tracking (MPPT) control of the PV module. Hence, it is necessary to decouple the pulsating power from input DC-bus line. Several papers have presented power decoupling topologies of grid interactive inverters for small distributed power generators. The authors have presented a novel AC module system which enhances the power decoupling. The feature of this system is that a flyback-type inverter is used and this enables AC current sharing among parallel connected inverters without current feedback control. However, above system has a serious drawback that the conversion efficiency is not high enough as expected because of its modulation method, the Sequential Magnetizing Modulation (SMM) method. Then this paper focuses on the modulation method, and proposes a novel modulation method, the Time-shared Magnetizing Modulation (TMM) method, in order to realize the power decoupling in the flyback inverter as well as high efficiency conversion. Figure 1 shows the main circuit configuration of the flyback inverter on the TMM method for the power decoupling, and the operation waveforms is depicted in Fig. 2. Since the decoupling capacitor, C X , deals only on the pulsating power, and C X supplies the pulsating power, then pure DC current flows from the PV module to DCbus line on the inverter. The DC-bus supplies the constant current and the surplus/lack of the magnetizing current is transferred to the Since the pulse width of output current, i 2 , on this method is wider than that on the SMM method, under the same switching frequency condition, the effective value of the magnetizing current can be reduced. Hence, it is possible to increase the conversion efficiency.The experimental results verify that the TMM method has advantage with respect to the conversion efficiency. This paper concludes that the flyback inverter on the TMM method for power decoupling is the most suitable for a single-phase grid interactive inverter for the AC module system because of its high efficiency operation.- MemberA photovoltaic AC module system has been expected to be installed on the private residences. The AC module inverter needs an active power d...

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